This invention relates to a binder for preparing toners which are used for developing electrostatic images formed in the process of electrophotography, electrostatography, electrostatic recording or the like.
Electrostatic images are generally developed with a toner, which occurs as a powder, to give toner images. These toner images are fixed either as they are or after transfer to a receiving paper sheet or the like.
Generally, toners are prepared by admixing a binder component and a coloring component, as necessary together with a charge control agent and other additives, melting the mixture with heating to achieve homogeneous admixture and, after cooling, comminuting the resultant mass to a desired particle size. The performance characteristics of a toner much depend on the binder component.
Various resins are known to be useful as binders for toner preparation. Recently, polyester binders have attracted much attention since they have high levels of negative chargeability and good low-temperature fixability, hence are suited for use in high-speed copying. Furthermore, they have good resistance to plasticizers. In other words, they will not cause prints or copies carrying images developed therewith to become indistinct when said prints or copies are placed in contact with a polyvinyl chloride sheet or film, for instance, and accordingly exposed to a plasticizer contained in said sheet or film.
Polyesters described as being useful as toner binders include, among others, the following: (i) Polymerization/esterification products from 1,4-benzenedicarboxylic acid and 2,2-dimethyl-1,3-propanediol as polymerized with 1,2,4-benzenetricarboxylic acid cyclic 1,2-anhydride (Japanese Kokai Tokkyo Koho No. 56(1981)-168660);
(ii) Nonlinear, low melting-point polyesters with an acid value of 10-60 as obtained from components comprising (A) an alkyl-substituted dicarboxylic acid and/or an alkyl-substituted diol, (B) a polycarboxylic acid having three or more carboxyl groups and/or a polyol having three or more hydroxyl groups, (C) a dicarboxylic acid and (D) an etherified diphenol (Japanese Kokai Tokkyo Koho No. 59(1984)-7960); and
(iii) Combinations of a polyvalent metal compound and an amorphous polyester having an acid value of 10-100 as obtained by reacting an etherified diphenol propoxylated and/or ethoxylated with a propoxy group content of not less than 50% with a phthalic acid or acids containing up to 40 mole percent of an aromatic carboxylic acid having three or more carboxyl groups (Japanese Kokai Tokkyo Koho No. 59(1984)-29256).
While, as mentioned above, toners in which polyester binders are used seen to be advantageous in respect of low-temperature fixability, practical use thereof encounters a problem, namely easy occurrence of the phenomenon called offset. This offset phenomenon includes staining of the surface of the receiving sheet supplied next due to transfer of a part of toner to the heat roller and, further, staining of the back of the receiving sheet with that portion of toner further transferred to the pressure roller in contact with the heat roller under pressure.
The low-temperature fixability and the prevention of offset are, in substance, contradictory requirements. Nevertheless, to prevent occurrence of the offset phenomenon while retaining the low-temperature fixiability, which is one of the characteristic features of polyester binders, is a problem to be solved by all means before putting to practical use toners in which polyester binders are used.
The inventions (i), (ii) and (iii) cited above as belonging to the prior art are to cope with the problem mentioned above but each has its drawbacks.
Thus, the binders according to the invention (i) have certain virtually definite or invariable physical properties since they are polymerization/esterification products from specific three components. Therefore, they have their limit in that they cannot meet varied requirements in the market concerning physical characteristics.
With toners in which the binders according to the invention (ii) are used, it is intended that the fixing temperature should be prevented from rising and at the same time the freeness from offset should be secured through introduction of the alkyl-substituted dicarboxylic acid and/or alkyl-substituted diol, which constitutes a soft segment, into the polyester skeleton. However, problems are encountered on the occasion of resin production. For example, considerable skill is required in producing said polyesters for acquiring a balance between both the performance characteristics.
Toners in which the binders according to the invention (iii) are used should secure high-speed fixability and offset resistance through the combined use of a polyvalent metal compound to overstep the limits of improvements on the polyester side. However, since the crosslinking is effected with an externally incorporated polyvalent metal compound, slight deviations in mixing conditions may result in variable performance characteristics. In some instances, the combined use of a metal compound may affect the charge, or static electricity, of the toner. Therefore, it is more advantageous to solve the above problem by an improvement in the binder polyester itself.
Furthermore, it is earnestly desired in the relevant industry to have another kind of binder than the binders according to the above-mentioned inventions (i), (ii) and (iii), which has both the low-temperature fixability and offset resistance, since the range of choice might be widened accordingly so as to cope with changes of toner design.
Under these circumstances, the present invention has now been completed to solve the above problems.